Input devices, including proximity sensor devices (such as touchpad and touch screen sensor devices) and biometric sensor devices (such as fingerprint sensor and eye scanner devices), are widely used in a variety of electronic systems. A sensor device typically includes a sensing region, often demarked by a surface, in which the sensor device detects one or more input objects to determine positional information and/or image the input object. Sensor devices may be used to provide interfaces for the electronic system. For example, sensor devices are often used as input devices for larger computing systems (such as notebook and desktop computers) and smaller computing systems (such as cellular phones and wearable computing devices) to provide interfaces for control inputs and user authentication.
Input devices utilizing optical sensors sometimes incorporate active illumination of the input object (e.g., a finger). These typically require additional auxiliary light sources that add to the cost and complexity of the device design and assembly.